huckster's — ((huckster: "retailer of small goods, in a petty shop or booth"; also: "a person ready to make his profit of anything in a mean or petty way" – OED.))

front door — in Jim's capacity as "schoolfellow", he has "front door status", i.e. the privilege of calling at the front door. In his capacity as "billing-boy", he has only "back door status", i.e. the duty to knock at the tradesman's entrance.

Settle bed

settle-bed — a bench that unfolds to a bed for sleeping. (("By day it functioned as a long seat or settle, and at night it could be unhinged outwards and down onto the floor, to make a floor-level double-bed, enclosed on all sides" – Kinmonth, Irish Rural Interiors in Art, 2008.)) …

Seamus Heaney Christmas card

Willed down, waited for, in place at last and for good.Trunk-hasped, cart-heavy, painted an ignorant brown.
And pew-strait, bin-deep, standing four-square as an ark.

If I lie in it, I am cribbed in seasoned deal
Dry as the unkindled boards of a funeral ship.
My measure has been taken, my ear shuttered up.

rugby — "Football, which, as a game, is worth ten of cricket, is played in short flannel knickers with perfectly plain ends, bare legs, heavy hob-nailed half-boots of chrome leather, and a jersey. It necessitates a complete change before and after play, especially after, as a serious player, particularly of the Rugby game, is liable to have little of him visible, for mud, when the whistle blows 'time.' A very complete bath, with soap, is therefore indicated. Football in the Winter, and cricket in the Summer, are the only games treated seriously at good-class schools." – The Boys' Outfitter, 1920.

claim a fight — pick a fight. ((Slang.ie.)) In Dublin schoolboy slang to "claim" someone = to claim the right to fight with that person. The person chosen has then been "claimed": to decline such a fight would be dishonourable. cf (("Peter, still standing on his bench, shouted: 'I claim a fight'" – Walpole, Fortitude, 1913.))

circle of honour — ? that group of onlookers which circles a fight, concealing, cajoling, and preventing retreat.

chaffed him — scoffed at him. ((chaff: "to banter, rail at, or rally, in a light and non-serious manner, or without anger, but so as to try the good nature or temper of the person 'chaffed'" slangy – OED.))

Grand Exhibit — punning "exhibit" (object on display) and "exhibition" (("an allowance or scholarship awarded to a student at a university or school" – Collins.))

goosed him — made a fool of him / hissed at him. ((goose: "befool" slang – OED.)) ((goose: "to condemn by hissing" theatrical and general – Partridge HS.))

piccaninnies — black children ((offensive – Collins.))

Presentation Missions — an inaccuracy: there were no Missions organized by the Presentation Brothers at this time, nor any to Africa until the 1960s. PresBros.org.

Gillray, "End of the Irish Invasion", 1797. "French warships, labelled Le Révolutionaire, L'Egalité and The Revolutionary Jolly Boat, being tossed about during a storm blown up by Pitt, Dundas, Grenville and Windham, whose heads appear from the clouds" – Wikipedia.

French scares – the fear, quite founded at the time (early 1800s), of revolutionary French invasions of – or fraternal assistance to – Ireland. cf Generals Humbert and Hoche.

spit and polish — (("the occupation of cleaning up or furbishing, as part of the work of a sailor or soldier; also in extended use, precise correctness, smartness; freq. as a derogatory expression in contrast with purposeful work or utility" – OED.)) jankers — punishment duty. ((services' slang – OED.)) Queen's Regulations — the British military code of conduct. ((Collins.)) "Queen's" because Mr Mack served under Queen Victoria.

forlorn hope — (("from Dutch verloren hoop, lost troop – a party of soldiers assigned to a particularly perilous duty" – Oxford Companion to Military History.)) (("with word-play or misapprehension of the etymology: a faint hope, a 'hope against hope'; an enterprise which has little chance of success" – OED.))

horny — hardened, calloused. ((OED.))

form — class or grade at school. ((OED.)) from the "form", or bench, pupils of similar abilities once shared. The term has connotations of fancy English boarding schools.

sniff of the glue-pot — ? the smell of seminal fluid. ((the glue-pot has come unstuck: "he gives off the odour of a genital exudation or of a seminal emission" low catchphrase: from ca. 1890 – Partridge HS.))